The object of the present invention is a measuring sensor of the type including a monolithic proof body provided with strain gauges attached to this proof body in at least one of its zones where an elastic deformation occurs as a result of a strain being applied to said proof body.
Generally, the proof body is comprised of a monolithic deformable rod which can be construed as a deformable parallelogram of which one end is fixed to a rigid frame and of which the other end is designed in such a manner as to receive the load to be measured.
The present invention is aimed at providing such a sensor, which could furthermore be used in a challenging or a corrosive environment, in salt water, in chlorinated water or in acid water, etc., where such strain gauges have to be protected to prevent their degradation.
Generally, strain gauges are protected by a coating or a film made of a material which is resistant to corrosive agents. Experience has however shown that it is always this protection which deteriorates in the first place, necessitating that the sensor be replaced, although its mechanical part (i.e. the proof body) is still sound.
Sensors have been proposed, in which the strain gauges are enclosed in sealed cavities, as is described, for example, in documents U.S. Pat. No. 4,488,611 or U.S. Pat. No. 5,220,971. These designs necessitate however the welding of metal covers or protective members on the proof body in the immediate vicinity of the deformation zones thereof, in order to encapsulate hermetically said strain gauges. This operation provides a durable protection for the strain gauges, but affects adversely the accuracy and the reliability of the measurements. In fact, the welding of these covers or protections in the immediate vicinity of the deformation zones of the proof body induces structural modifications in the material constituent of the proof body in these zones, thereby modifying, in particular, its elasticity characteristics, which leads to inaccuracies or measurement errors, which cannot be readily corrected electrically.
The present invention is aimed at providing a sensor which includes a monolithic proof body with strain gauges which are encapsulated, and hence protected from the environment, and which obviates the above-mentioned drawbacks, namely which is durable and in which the protection of the gauges does not influence the measuring characteristics.
The object of the present invention is a measuring sensor including a monolithic proof body which is provided with strain gauges and which has the characteristic features set out in claim 1.